1. Field of the Invention
The present invention provides a non-faceted nanoparticle reinforced metal matrix composite having increased ductility, while maintaining strength. In particular, a non-faceted nanoparticle reinforced metal matrix composite is provided comprised of spherical or ellipsoidal shaped (non-faceted) nanoparticles comprising one or more of boron carbide, titanium diboride, silicon nitride, alumina and boron nitride, and a nanostructured matrix composite comprised of one or more metals and/or metal alloys. In addition, a method of manufacturing such a non-faceted nanoparticle reinforced metal matrix composite is provided.
2. Description of the Related Art
Particulate reinforced aluminum matrix composites have high strength, high modulus, lightweight, good performance at high temperature, excellent fatigue resistance, creep resistance and abrasion resistance. The properties of the matrix composites are determined by the reinforcing candidates and by the microstructure of the matrix. Boron carbide (B4C) is a good reinforcing candidate for the aluminum matrix composite, ranking third in hardness (just after diamond and cubic boron nitride) and having a low density of 2.51 g/cm3 (lighter than Al). The microstructure of the matrix is also important to the overall properties of the composites, such as the gain size of the matrix material. For example, for a monolithic metal alloy, the strength of the alloy increases with the decreasing of the grain size. Thus, if a nanostructured Al matrix is achieved, the strength of the composite material can be further improved.
Al-based composite with the nanostructured Al matrix reinforced by the hard B4C particles can be fabricated via cryomilling. Nanostructured material is material with a microstructure the characteristic length of which is on the order of a few (typically 1-550) nanometers. Microstructure refers to the chemical composition, the arrangement of the atoms (the atomic structure), and the size of a solid in one, two, or three dimensions. Nanostructured materials have received increasing attention due to their superior physical and mechanical properties. They are used in the electronic industry, telecommunication, electrical, magnetic, structural, optical, catalytic, drug delivery, and in consumer goods.
Nanostructured materials have generally conventionally been produced by (1) powder metallurgy, (2) deposition to bulk nanostructured materials, and (3) structural refinement by severe plastic deformation. In powder metallurgy processes, nanostructured materials are commonly made via mechanical milling of powder and subsequent consolidation of the powder into bulk material. There are several disadvantages with this approach. For example, contamination is unavoidable during consolidation.
Modification of these methods, however, can lead to the development of processes that are more practical. For instance, it has been reported that mechanical milling under liquid nitrogen can prevent the powders from being severely oxidized from air, and small nitride or oxy-nitride particles, which are within the size of 2-10 nm, are produced in-situ during milling. These inclusions (including precipitates, dispersoids and constituent particles), as they are called, can both strengthen the metal and enhance the thermal stability (i.e., control the gain growth) of the nanostructured materials. As another example, if the temperature and/or period to consolidate nanostructured powders into fully dense bulk materials can be reduced, severe grain growth can be suspended and thus the nanostructure can again be retained.
With chemical processes, nanostructured materials are created from a reaction with organometallics that precipitate particles of varying sizes and shapes. The process can, however, introduce excess carbon and/or nitrogen into the final composition. An alternative approach is the solution-gelation (sol-gel) process where ceramic product ion is similar to organometallic processes, except sol-gel materials may be either organic or inorganic. Both approaches involve a high cost of raw materials and capital equipment, limiting their commercial acceptance.
Physical or thermal processing involves the formation and collection of nanoparticles through the rapid cooling of a supersaturated vapor (gas phase condensation, see U.S. Pat. No. 5,128,081). Thermal processes create the supersaturated vapor in a variety of ways, including laser ablation, plasma porch synthesis, combustion flame, exploding wires, spark erosion, electron beam evaporation, sputtering (ion collision). In laser ablation, for example, a high-energy pulsed laser is focused on a target containing the material to be processed. The high temperature of the resulting plasma (greater than 10,000° K) vaporizes the material quickly allowing the process to operate at room temperature. The process is capable of producing a variety of nanostructured materials on the laboratory scale, but it has the disadvantage of being extremely expensive due to the inherent energy inefficiency of lasers, and, therefore, is not suitable for industrial scale production.
Mechanical milling has been widely used to fabricate nanostructured metal powder and powder for metal matrix composites. However, it can be difficult to obtain nanostructured aluminum alloys with conventional mechanical milling, because of the high recrystallization rate due to the low melting temperature of aluminum. Cryogenic milling or cryomilling is a modified mechanical milling technique where the mechanical milling is carried out at cryogenic temperatures, usually in liquid nitrogen or a similar chilled atmosphere. Cryomilling has been employed to successfully fabricate nanostructured aluminum alloy powders and powders for aluminum metal matrix composites, which exhibit good thermal stability, because the cryogenic temperature retards the recovery of the aluminum. Strain is accumulated during cryomilling, leading to dislocation activity, ultimately causing the formation of nanoscaled grains within the cryomilled powder.
The combined effect of the ultra-fine dispersion of particles formed during cryomilling and the reduced grain size is a powder that can be used to make a bulk material with relatively high strength. This type of material will also exhibit better creep resistance compared to its conventional counterpart. It has been reported that cryomilled aluminum alloys and aluminum metal matrix composite powders have nanoscaled structures with very good thermal stability. Also, cryomilling can be easily scaled up to produce tonnage quantities. Thus, cryomilling is one of the few processing approaches available for the fabrication of large quantities of nanostructured metal powders.
The nanostructured powders described above must be consolidated into bulk materials. Traditional consolidation approaches, such as hot pressing (HP), hot isostatic pressing (HIP), and cold isostatic pressing (CIP) have been employed for consolidation into bulk materials. U.S. Patent Application Publication No. 2004/0065173 to Fritzmeier et al. discloses aluminum alloy produced by blending aluminum with two other metals by cryomilling. The cryomilled alloy is subsequently consolidated by HIP. These consolidation methods require other gases to improve the efficiency of consolidation. Further, these traditional consolidation approaches need very high pressures, on the order of GPa, which is provided by high-pressure argon, and long cycle time that can last for several hours.
Cryomilling has been demonstrated to provide a homogeneous distribution of the B4C in the Al matrix and a good interface between the B4C and the Al matrix. However, the B4C/Al nanocomposite fabricated by cryomilling and subsequent consolidation approach exhibited limited ductility despite the high strength. Therefore, there is a need for a strong, yet highly ductile reinforced metal matrix composite, and a process for making such particulate reinforced matrix composites.